The present invention relates to a circuit for providing saw-tooth current with a trace and retrace interval in a coil, in particular, a deflection coil of a television kinescope, in which the said coil is connected to a trace and retrace condenser with which it forms a resonant circuit; in which a first diode is connected in parallel to the said resonant circuit with such polarity that it is made conductive by the saw-tooth current during a first part of the trace interval; in which the said resonant circuit is also connected, through a second diode, to a controllable switch, with a control electrode, connected to a source of periodic signals which make it conductive at least during the last part of the trace interval, the said second diode being connected with such polarity that it is made conductive by the saw-tooth current during the last part of the trace interval; in which the said controllable switch is connected to a supply voltage source through a first inductor and, finally, in which part of the energy stored in the said first inductor, during the interval in which the said controllable switch is conductive, is transferred, during the retrace interval, to the said resonant circuit through a third diode.
A circuit of this type has already been described by A. Farina and G. Zappala (see, for example, FIG. 1 on page 237 of the "Elettronica e Telecomunicazioni" magazine No. 6-1976).
The said known circuit allows obtaining in the deflection coil a saw-tooth current which is able to produce a deflection of the electronic beam in a television kinescope; the amplitude of the current can be easily controlled by varying the conduction time of the transistor which forms the controllable switch, i.e. the length of the positive pulse applied to the base of the transistor.
However, in this known circuit a certain amount of power must be dissipated in the load resistor which, moreover, cannot be connected directly to the negative pole of the supply battery, which makes it difficult for this power to be used.
In the 1976 No. 6 issue of the aforementioned magazine, a number of variations to the circuit are suggested by which one terminal of the load resistor can be connected to the negative pole of the battery, these variations are rather complex, however, (requiring the use of more diodes), i.e. they lead to an increase in the current of the transistor for the same value of current in the deflection coil. Furthermore, the polarity of the voltage which can be obtained at the terminals of the load resistor is the opposite of that of the battery voltage which often poses practical problems, for example, in the case of supplying auxiliary circuits on a television set. Finally, if there are variations in the losses associated with the deflection coil (which occur, for example, on a television set when the beam current is varied if, as is usually the case, the extra-high voltage is obtained from the retrace or flyback pulse), also the voltage at the terminals of the said load resistor varies.